556 



LOWEST VERTEBRATES. 



Cephalaspis. 



and if it was not for the invariable absence of limbs and jaws, and the forward 

 position of the breathing apparatus in Pteraspis and its allies, these animals might 

 be placed, without hesitation, in the class of fishes. The possibility that limbs and 

 jaws were present, but not calcified enough to be preserved, must, however, be 

 borne in mind; while the negative evidence on this subject, and the want of 

 information as to the nature of the tail, are factors necessitating caution in the 

 determination of affinities." 



The next family of the group is typified by the genus Cephalaspis, 

 in which the front shield appears to be confined to the head and gill- 

 region, and consists of a single piece, rounded or pointed in front, abruptly 

 truncated behind, and with the rounded margin bent inwards below to form an 

 ornamented flattened rim. Of the triple-layered shield, the inner layer is bony, 

 the thick middle one solid, although traversed by a network of blood-vessels, while 

 the upper one is tuberculated and resembles teeth in structure. The eyes are placed 

 close together in the middle of the shield, the nostrils must have had much the 

 same position as in Pteraspis, and at the back of the shield there occurs on each 

 side a small flap which must be regarded as a gill-cover. Immediately behind the 



shield commences the 

 ordinary scaling of 

 the body, without any 

 signs of arches for the 

 support of limbs. 

 Paired fins appear, 

 indeed, to be totally 

 absent, although a dorsal and a caudal fin, stiffened by little elongated scales in 

 place of rays, are present. The large, deep, quadrangular scales covering the body 

 form a series of interlocking rings, doubtless corresponding in the living state to 

 the underlying muscle-plates of the body. 



The third modification of the group, as represented by the 

 Devonian Pterichthys, agrees in the general structure of the shield 

 with certain members of the last section in which there is no dividing line between 

 the head-shield and the united scales of the body. The head is, however, sharply 

 defined from the body ; and the armour, instead of being simple, consists of a 

 number of overlapping plates arranged symmetrically to one another. An 

 important point of distinction from 

 all the preceding forms is to be 

 found in the presence of a pair of 

 hollow limb-like pectoral append- 

 ages, jointed near the middle. A 

 small movable plate between the 

 eyes seems to have lodged a median 

 eye ; another movable plate on the 

 cheek appears to represent the gill- 

 cover ; and a pair of loose jaw-plates on the lower surface of the front of the head, 

 in some forms at least, are finely toothed on the hinder border; but nothing 

 definite is known with regard to the nature of the nose, mouth, and jaws. 



restoration OF Cephalaspis. (From Woodward, Cat. Foss. Fish. Brit. Mus.) 



Pterichthys. 



RESTORATION OF pterichthys. (From Traquair.) 



